Safety helmets are widely used by motorcyclists, auto racers, bike riders, athletes, and others who need to wear protective headgear. These helmets typically comprise a tough outer shell and an internal shock absorbing liner such as, for example, a foam plastic. The outer shell is substantially rigid, but when subjected to the high impact experienced in a collision, buckles momentarily, sometimes compressing the foam plastic that absorbs the energy of the collision. Once compressed the foam plastic liner remains compacted, and thereafter is not as capable of absorbing the energy of another collision. The outer shell usually has sufficient flexibility to return to its normal condition and the damage to the shock absorbing liner is visually undetectable. In other words, one impact may render the helmet incapable of thereafter adequately protecting the user, but the degradation of the shock absorbing character of the liner is unnoticed. Although one high-energy impact may render the helmet incapable of adequately protecting the user, several repeated low energy impacts may also render the helmet incapable of adequately protecting the user. Manufactures of various types of helmets fail to recognize that such repeated low energy impacts degrade the ability of the helmet to protect the user. Consequently, there is a risk of an athlete in a contact sport such as, for example, lacrosse, hockey, football, skiing, snow boarding, etc., sustaining a serious head injury because the helmet has lost its ability to absorb the energy of even a low level impact due to sustaining a number of low level impacts. Moreover, soldiers and other military personnel, frequently experience brain injuries from bomb blasts even when wearing a helmet, but are unaware of such injury.